Under bump metallurgy for Lead-Tin bump over copper pad

ABSTRACT

The present invention describes a method including providing a component, the component having a bond pad; forming a passivation layer over the component; forming a via in the passivation layer to uncover the bond pad; and forming an under bump metallurgy (UBM) over the passivation layer, in the via, and over the bond pad, in which the UBM includes an alloy of Aluminum and Magnesium. 
     The present invention also describes an under bump metallurgy (UBM) that includes a lower layer, the lower layer including an alloy of Aluminum and Magnesium; and an upper layer located over the lower layer.

This is a Divisional Application of Ser. No.: 10/262,281 filed Sep. 30,2002 now U.S. Pat. No. 6,703,069.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor integratedcircuit (IC) manufacturing, and more specifically, to a method offorming a more reliable under bump metallurgy (UBM) and an UBM that ismore reliable.

2. Discussion of Related Art

Chip-to-package interconnections have traditionally involved wirebondingwhich is very cost-effective. Wirebonding is the use of very fine metalwires to join contacts on a chip with corresponding contacts on apackage. As the size of a transistor is reduced, the size of thechip-to-package interconnection also has to be scaled down. However, theperformance and the reliability of the chip-to-package interconnectionmay be affected since wirebonding requires the routing of all theinput/output (I/O) connections to the edges of the chip.

Solder bumping is the use of reflowable solder balls to join contacts onthe chip with corresponding contacts on the package. Solder bumpingrequires that the chip be flipped over to face the package. Solderbumping permits I/O connections to be placed across the surface of thechip, which results in many advantages. First, bumping significantlyincreases the density of the I/O connections. Second, bumping simplifiesthe design and layout of the chip. Third, bumping decreases thefootprint of the package. Fourth, bumping greatly enhances thereliability of the I/O connections.

Transistors on the chip have traditionally been connected with Aluminumlines. As the size of the transistor continued to be reduced, Copper wasintroduced as a replacement for Aluminum. Copper has lower resistivitythan Aluminum so performance of the chip is improved. Copper is moreresistant to electromigration than Aluminum so reliability of the chipis also improved.

The bump attached to the bond pad of the chip has traditionally beenformed with a Lead-Tin Solder. However, during thermal cycling, the Tinin the bump tends to migrate through cracks or other defects in the UBMand react with the Copper in the bond pad to form an intermetalliccompound which may cause shorting of the interconnects thereby leadingto premature failure of the interconnect.

Thus, what is needed is a method of forming a more reliable under bumpmetallurgy (UBM) and an UBM that is more reliable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are illustrations of an elevation view of an embodiment of amethod of forming a more reliable under bump metallurgy (UBM) accordingto the present invention.

FIG. 1E is also an illustration of an elevation view of a more reliableUBM, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following description, numerous particular details, such asspecific materials, dimensions, and processes, are set forth in order toprovide a thorough understanding of the present invention. However, oneskilled in the art will realize that the invention may be practicedwithout these particular details. In other instances, well-knownsemiconductor equipment and processes have not been described inparticular detail so as to avoid obscuring the present invention.

The present invention includes a method of forming a more reliable underbump metallurgy (UBM) and a UBM that is more reliable. The method of thepresent invention suppresses the diffusion of Tin from a solder bump toan underlying Copper bond pad, minimizes the formation of a Copper-Tin(Cu:Sn) intermetallic compound, and prevents shorting of interconnects.The UBM of the present invention may include Aluminum with an alloyingelement such as Magnesium.

An embodiment of a method of the present invention is shown in FIGS.1A-1E. A component 100 may include a substrate 105, in which a device,such as a transistor, has been formed from a semiconducting material,such as Silicon, an insulating material, such as Silicon Oxide andSilicon Nitride, and a conducting material, such as doped Polysiliconand Copper.

The transistor in the substrate 105 may include interconnects that havebeen formed from multiple layers of conducting lines which are isolatedby insulating material. The conducting lines on the different layers maybe connected by conducting plugs in vias formed through the insulatingmaterial. The conducting lines and plugs may be formed from the same ordifferent materials, such as Copper metal or alloy. The insulatingmaterial may be an interlayer dielectric (ILD) formed from Silicon Oxideor a low dielectric constant material, such as a porous Carbon-dopedOxide (CDO or SiOC).

A landing pad, or bond pad 110, may be formed over the substrate 105 toprovide access to the interconnects of the underlying device. The bondpad 110 may permit Input/Output (I/O) of an electrical signal, power, orground, to and from the underlying device, such as the transistor,through the interconnects. The bond pad 110 may be formed from Coppermetal or alloy.

As shown in an embodiment of the present invention in FIG. 1A, apassivation layer 120 may be formed over the substrate 105 to keep outcontaminants and moisture and prevent corrosion or other damage to theinterconnects and the underlying device. The passivation layer 120 mayalso serve as a planarizing layer to assist in subsequent processing,especially photolithography. The passivation layer 120 may further serveas a stress buffer layer. The characteristics that are desired for thepassivation layer 120 include good adhesion, good thermal stability,high tensile strength, and good chemical resistance.

The passivation layer 120 may include Silicon Oxide, Silicon Nitride,and an organic material, such as a polyimide. In one embodiment, thepolyimide may be covered with a radiation-sensitive material, such as aphotoresist, and patterned with photolithography. In another embodiment,a photodefinable polyimide, may be spin-coated directly over the bondpad 110 and the substrate 105. The photodefinable polyimide may beexposed to radiation of the appropriate wavelength, energy, and dose, asmodulated by a mask with a bump pattern. Developing of thephotodefinable polyimide followed by etching to uncover the bond pad 110will form a via 125 over the bond pad 110, as shown in an embodiment ofthe present invention in FIG. 1B.

In order to prevent high contact resistance, a plasma pre-clean, orashing, may be performed to remove any Oxide that may be present on thebond pad 110. Then, an UBM 130 may be formed over the passivation layer120, in the via 125, and over the bond pad 110, as shown in anembodiment of the present invention in FIG. 1C.

The UBM 130 provides a reliable electrical and mechanical interfacebetween the underlying bond pad 110 and the overlying bump 155. In anembodiment of the present invention, the UBM 130 may include amultilayer stack of materials.

In one embodiment of the present invention, the UBM 130 may include anupper layer 136 located over a lower layer 133. In an embodiment of thepresent invention, the upper layer 136 of the UBM 130 may be formed froma Nickel-Vanadium (NiV) alloy which is wettable by the solder 150. Amaterial that is wettable by the solder 150 will dissolve in the solder150 to strengthen the metallurgical bond. In an embodiment of thepresent invention, the upper layer 136 of a NiV alloy may have athickness of about 50-1,000 nm.

In another embodiment of the present invention, the lower layer 133 ofthe UBM 130 may be formed from a stack of materials, including anAluminum alloy that is sandwiched between a top Titanium and a bottomTitanium. The top Titanium improves adhesion between the upper layer 136and the lower layer 133 of the UBM 130. The top Titanium also suppressesdiffusion of Nickel (Ni) from the upper layer 136. In an embodiment ofthe present invention, the top Titanium may have a thickness of about20-500 nanometers (nm).

The Aluminum alloy in the stack may include Aluminum and one or moresuitable alloying elements. An alloy is a solid solution of two or moremetals. A suitable alloying element will suppress the diffusion of Tinfrom the overlying bump 155. In an embodiment, the suitable alloyingelement may be about 0.5-2.5% Magnesium by weight. In other embodimentsof the present invention, the suitable alloying element may includeChromium (Cr), Germanium (Ge), Hafnium (Hf), Lithium (Li), Manganese(Mn), Palladium (Pd), Vanadium (V), and Zirconium (Zr). In general,Titanium (Ti) and Silicon (Si) are not suitable alloying elements forthe Aluminum. In one embodiment of the present invention, the Aluminumalloy may have a uniform composition through its thickness. In anotherembodiment of the present invention, the Aluminum alloy may have agraded composition through its thickness. In one embodiment of thepresent invention, the Aluminum alloy may have a thickness of about100-1,000 nm. In another embodiment of the present invention, theAluminum alloy may have a thickness of about 100-400 nm.

In one embodiment of the present invention, the Aluminum alloysuppresses diffusion of Tin (Sn) from the overlying bump 155. In anotherembodiment of the present invention, the Aluminum alloy suppressesdiffusion of Copper (Cu) from the underlying bond pad 110. The formationand growth of Copper-Tin (Cu:Sn) intermetallic compounds is controlledand limited to prevent shorting of interconnects.

The bottom Titanium improves adhesion between the lower layer 133 of theUBM 130 and the underlying bond pad 110. Titanium decreases interfacialcontact resistance to the bond pad 110 by chemically reducing any oxidethat may be present on the bond pad 110. The edges of the via 125 formedthrough the passivation layer 120 should be hermetically sealed by theUBM 130 to prevent corrosion of the interconnects that are located belowthe bond pad 110. Titanium also increases resistance toelectromigration. In an embodiment, the bottom Titanium may have athickness of about 20-500 nm.

The UBM 130 may be formed as a blanket film by physical vapor deposition(PVD) or sputtering. In one embodiment of the present invention, the UBMmay be formed by ionized PVD (I-PVD) to achieve good conformality infilling a via 125 having a high aspect ratio.

If two or more layers are being sputtered sequentially for the UBM 130,a different target may be used for each layer. The sequential sputteringmay be done without breaking vacuum so as to reduce contamination,prevent formation of Oxides, and improve throughput.

If two or more materials, such as Aluminum and Magnesium, are to beco-sputtered for a layer in the UBM 130, a particular composition of thesputtering target may be specified in order to produce the desiredcomposition in the layer of the UBM 130 being formed. The difference incomposition between the target and the layer may be caused by adifference in sputtering efficiency or sticking coefficient.

In another embodiment of the present invention, the composition of alayer that has been sputtered may be modified by annealing. Theannealing may be performed in a gas. The gas may be inert or reactive.

Next, the UBM 130 may be covered with a layer of photoresist 140. Afteraligning the component 100 with respect to a mask, the photoresist 140may be exposed with the appropriate radiation. After developing thephotoresist 140, an opening 145 is formed in the photoresist 140 overthe upper layer 136 of the UBM 130, as shown in an embodiment in FIG.1D. In one embodiment, the opening 145 in the photoresist 140 may belocated over the bond pad 110. In another embodiment, the opening 145may be offset to one side of the bond pad 110.

An electroplating cell includes two electrodes that are immersed in anelectrolyte and connected through an external circuit to a power supply.A consumable anode in the electroplating cell may include an alloy ofthe metals which form the solder 150. In one embodiment, the solder 150may be a high Lead solder, having a composition of 95% Lead (Pb) and 5%Tin (Sn), by weight. The external circuit may remove electrons from theanode to oxidize the metals and release positively-charged metal ionsinto the electrolyte.

The UBM 130 may serve as a cathode in the electroplating cell. The UBM130 provides a low-resistance electrical path for the external circuitto supply electrons to reduce the positively-charged metal ions in theelectrolyte and electroplate the metals, through the opening 145 in thephotoresist 140, over the upper layer 136 of the UBM 130.

The solder 150 will spread out in a mushroom shape once the thickness ofthe solder 150 being electroplated in the opening 145 of the photoresist140 exceeds the thickness of the photoresist 140, as shown in anembodiment of the present invention in FIG. 1D.

In another embodiment of the present invention, a very thick layer ofphotoresist 140 is used so the thickness of the solder 150 beingelectroplated in the opening 145 of the photoresist 140 does not exceedthe thickness of the photoresist 140. As a result, the solder 150retains a pillar shape within the opening 145 in the photoresist 140.

After the solder 150 has been electroplated into mushroom-shaped, orpillar-shaped, islands on the component 100, the photoresist 140 isstripped off.

In order to prevent shorting of the islands of electroplated solder 150,a wet etch solution, that will not etch the solder 150, may be used, inone embodiment of the present invention, to etch away the exposedportion of the UBM 130 that is not covered by the solder 150. Etchingaround the edges of the covered portion of the UBM 130 located beneaththe islands of solder 150 should be controlled so any undercut isminimized, such as to 2 microns (um) or less.

After etching away the exposed portion of the UBM 130, the islands ofsolder 150 may be reflowed. The melting (liquidus) temperature of thesolder 150 depends on the alloy composition of the solder 150. A highLead solder, such as 95 Pb/5 Sn by weight percent, may flow at about 308degrees Centigrade.

A convection oven may be used to reflow the solder 150. Forming gas maybe used as a cover gas in the convection oven. Forming gas may have apassive component, such as 90% Nitrogen to prevent formation of Oxides,and an active component, such as 10% Hydrogen to chemically reduceexisting Oxides.

Upon cooling, surface tension will draw each island of solder 150 into abump 155 having an approximately spherical shape, as shown in anembodiment of the present invention in FIG. 1E. The minimum distancebetween the centers of adjacent islands of solder 150, or bump pitch, islimited by assembly and reliability considerations and may be selectedfrom a range of about 50-300 um.

After solidification, the bump 155 may have a diameter selected from arange of about 20-150 um. The diameter and height of the bump 155depends on the area of the wettable metal base, which is determined bythe undercut of the UBM 130. The thickness, or bump 155 height, shouldbe well-controlled, with a standard deviation of less than 2.5 um withinthe component 100 and across a batch of components 100.

The bump 155 height will affect the standoff height when the component100 is later attached to a substrate in a package. The uniformity ofbump 155 height across the component 100 will also affect thecoplanarity of the bumps 155 across the component 100. Coplanarity, inturn, determines how reliably all of the bumps 155 on the component 100may be subsequently connected to the pads on the substrate in thepackage.

FIG. 1E also shows a more reliable UBM 130, according to an embodimentof the present invention. In one embodiment of the present invention,the UBM 130 may include a multilayer stack of materials, such as a lowerlayer 133 and an upper layer 136.

In one embodiment of the present invention, the lower layer 133 of theUBM 130 may include an Aluminum alloy. The Aluminum alloy may includeone or more suitable alloying elements, such as Magnesium. The suitablealloying elements suppress the diffusion of metals, such as Tin (Sn),from the overlying bump 155. In an embodiment of the present invention,the Aluminum alloy may include about 0.5-2.5% by weight of Magnesium. Inone embodiment of the present invention, the Aluminum alloy may have auniform composition through its thickness. In another embodiment of thepresent invention, the Aluminum alloy may have a graded compositionthrough its thickness. In one embodiment of the present invention, theAluminum alloy may have a thickness of about 100-1,000 nm. In anotherembodiment of the present invention, the Aluminum alloy may have athickness of about 100-400 nm.

In another embodiment of the present invention, the lower layer 133 ofthe UBM 130 may include a stack of Titanium/Aluminum alloy/Titanium. TheTitanium underlying the Aluminum alloy may have a thickness of about20-500 nm. The Titanium overlying the Aluminum alloy may have athickness of about 20-500 nm.

The upper layer 136 of the UBM 130 may include a Nickel-Vanadium (NiV)alloy. The NiV alloy may have a thickness of about 50-1,000 nm.

Many alternative embodiments and numerous particular details have beenset forth above in order to provide a thorough understanding of thepresent invention. One skilled in the art will appreciate that many ofthe features in one embodiment are equally applicable to otherembodiments. One skilled in the art will also appreciate the ability tomake various equivalent substitutions for those specific materials,processes, dimensions, concentrations, etc. described herein. It is tobe understood that the detailed description of the present inventionshould be taken as illustrative and not limiting, wherein the scope ofthe present invention should be determined by the claims that follow.

Thus, we have described a method of forming a more reliable under bumpmetallurgy (UBM) and an UBM that is more reliable.

1. An under bump metallurgy (UBM) comprising: a lower layer disposedover a bond pad, said lower layer comprising an alloy of Aluminum andMagnesium to suppress diffusion of metals; and an upper layer disposedover said lower layer to wet solder from an overlying bump.
 2. The UBMof claim 1 wherein said alloy comprises about 0.5-2.5% by weight ofMagnesium.
 3. The UBM of claim 1 wherein said alloy comprises athickness of about 100-1,000 nanometers.
 4. The UBM of claim 1 whereinsaid lower layer further comprises a bottom Titanium underlying saidalloy.
 5. The UBM of claim 1 wherein said lower layer further comprisesa top Titanium overlying said alloy.
 6. The UBM of claim 4 wherein saidbottom Titanium underlying said alloy comprises a thickness of about20-500 nanometers.
 7. UBM of claim 5 wherein said top Titanium overlyingsaid alloy comprises a thickness of about 20-500 nanometers.
 8. The UBMof claim 1 wherein said upper layer comprises a Nickel-Vanadium alloy.9. The UBM of claim 8 wherein said Nickel-Vanadium alloy comprises athickness of about 50-1,000 nanometers.